Are you already familiar with our industrial-grade accredited inspection services?

� Accredited laboratory in line with DIN EN ISO / IEC 17025, to qualify and validate new non­destructive testing (NDT) processes for industrial applications

� Accelerated time-to-market and opportunity for qualified, norm-compliant deployment in industri­al applications as well as for new in­house developments or custom adaptation of innovative NDT technologies, even in fields where norms have not been established

� Certification of the corresponding quality management system in accordance with DIN EN ISO 9001

Fraunhofer-Institut für Zerstörungsfreie Prüfverfahren IZFP

Campus E3 166123 Saarbrücken

+49 681 9302 0

info@izfp.fraunhofer.dewww.izfp.fraunhofer.de

STRESS MEASUREMENTS BY ULTRASOUND

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F R A U N H O F E R I N S T I T U T E F O R N O N D E S T R U C T I V E T E S T I N G I Z F P

Tensile test and result representation

Results (example)

The adjacent figure represents the in situ application of EMAT on a cyclically loaded sample including (down right) the results of the time-of-flight (tof) measurement in relation to the occuring tensile and pressure stresses (top right).

The results of the measurements prove a good correlation to the corresponding fatigue processes in the loaded sample. Applied tensile stresses result in solidification processes, intrusions and extrusions at the sample surface and martensite formation, all of them leading to an increase of tof. Pressure stresses, however, result in softening processes redounding to a decrease of tof. Thus, a sample breakdown can be spotted early by an increase of tof. Moreover, this project was the first one worldwide to successfully monitor fatigue tests by integrated EMAT technology at temperatures of up to 300 °C and to document the results in realtime.

Further applications

Applications are found wherever existing infrastructure is subject to ageing or to possible damage so that a periodic monitoring is important to prevent physical injury or loss of property. Additionally, our technology is applied for quality control immediatly after the production process. This comprises amongst others

� Measurement of relative and absolute stress values � Distinction between load and residual stresses � In situ stress measurements

Situation

In the volume of virtually any metallic material‘s micro-texture so-called lattice defects (imperfections, inclusions, voids, dislocations) crop up. The texture state is crucial for the material‘s behavior and is subject to long­term load­based changes all over the life cycle of components such as temperature or constant and changing mechanical loads. Thus, in case of safety­related components the early detec­tion of material changes and stress states is an essential constituent of the component‘s reliability of operation.

Solution

Inspection by ultrasound is an eligible procedure to evaluate both, residual stresses and incipient load­ induced changes of materials and stresses in metals. Basically, all tension and strain changes in materials are subject to the so­called acousto­elastic effect which raises measurable changes regarding the velocity of propagation of ultrasound waves. Hence, measurements of ultrasonic velocity of propagation are used to detect emerging material and stress changes.

Depending on the applied wave modes (longi­tudinal, transversal (SH, SV)) and their propagation and polarization direction in relation to the stress direction additional changes of the measured ultrasonic time-of-flight occur (see figure). According to the inspection conditions conventional piezo­electric probes or electromagnetically excited ultrasound transducers (EMAT) are applied. Due to the excitation mechanics of the ultrasound wave EMAT is suited but for the inspection of ferromagnetic and/or electrically conductible materials.

Determination of circumferential and radial residual stresses before (top left) and after (down right) hardening of a cylindric gearing component

Relative change of sound velocities of different wave and polarization modes as a function of strain

In situ measurements by EMAT (collective operatio-nal load)

Change of sound velocity of a transmitter / receiver setup as a function of the applied strain of a tensile test

Ultrasonic time-of-flight [µs] as a function of the applied strain [MPa]Transmitter / receiver setup